This application is based on an application No. 11-281322 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube used in television sets, computer displays and the like, and in particular to an apparatus for correcting convergence in a color cathode ray tube (hereafter CRT) that corrects raster distortion using magnets.
2. Related Art
One method used to correct convergence in a color CRT that uses an inline electron gun is a self-convergence method. This method corrects convergence involving pincushion distortion of the horizontal deflection field and barrel distortion of the vertical deflection field. The self-convergence method enables apparatuses with a simple construction and an excellent cost-performance ratio to be manufactured, and is consequently in widespread use.
In a conventional color CRT using the self-convergence method, for example a color CRT with a deflection angle of 90xc2x0, and a large screen curvature, the vertical deflection field experiences barrel distortion, thereby causing the horizontal component (hereafter referred to as xe2x80x98Bhxe2x80x99) of the vertical deflection field to become larger nearer to the right and left edges of the CRT. FIG. 1A is a graph plotting Bh against a horizontal axis H of the CRT. If a central point along the horizontal direction of the CRT is taken as an origin O, line 1 showing Bh is symmetrical about the origin O, and slopes upward more steeply the further it is from the origin O.
According to Fleming""s Law, the vertical deflection force applied to the electron beams will increase as Bh becomes larger. Therefore, in a color CRT using the self-convergence method, electron beams passing closer to a vertical axis V will receive a weaker vertical deflection, and electron beams passing further away from the vertical axis V will receive a stronger vertical deflection. When an inline electron gun is used, three electron beams corresponding to the three colors RGB (red, green and blue) are horizontally aligned, so that, if we ignore a case in which the central beam of the three electron beams coincides with the vertical axis V, there will be some variations in the vertical deflection force applied to electron beams on either side of the vertical axis V. FIG. 1B shows vertical deflection forces Fr, Fg and Fb, received respectively by the red, green and blue electron beams R, G and B. Electron beams emitted by an inline electron gun are usually arranged in the order B, G and R from left to right as seen from in front of the screen. In this specification, it is assumed that all electron beams are arranged in this order. When the electron beam G coincides with the vertical axis V, in other words when it is positioned so as to correspond to the origin O of the horizontal axis H, vertical deflection forces Fr and Fb are equal, and vertical deflection force Fg is smaller than both vertical deflection forces Fr and Fb. When the electron beam R is further away from the origin O than the electron beam B, however, the vertical deflection forces received by the electron beams are such that Fb less than Fg less than Fr. Conversely, when the electron beam B is further away from the origin O than the electron beam R, the vertical deflection forces received are such that Fb greater than Fg greater than Fr.
As a result, when horizontal magenta lines are displayed at the top and bottom edges of the screen, the misconvergence shown in FIG. 2 is caused. Here, a red component R (the solid line in the drawing) and a blue component B (the broken line in the drawing) in each magenta line on a display screen 2, diverge vertically towards the corners of the screen. Since Bh is largest when the amount of vertical deflection reaches its maximum, this misconvergence is particularly marked at the corner areas of the screen. This type of misconvergence is hereafter referred to as PQV pincushion pattern misconvergence.
Japanese Laid Open Patent 8-98193 discloses a color CRT that corrects PQV pincushion pattern misconvergence by weakening the barrel distortion of the vertical deflection field. FIG. 3A is a graph plotting the values of Bh, both before and after barrel distortion of the vertical deflection field has been weakened, against the horizontal axis H. As a result of weakening barrel distortion, the variation in Bh changes from line 1 to line 3 in the drawing. Thus, as shown in FIG. 3B, the variations in Bh along the horizontal are reduced, and PQV pincushion pattern misconvergence is corrected.
If the barrel distortion of the vertical deflection field is weakened, this in turn weakens the ability of the CRT to correct misconvergence using a self-convergence method. Here, if a magenta line is displayed vertically down the center of the display screen 2, the misconvergence shown in FIG. 4 will be generated. This misconvergence is hereafter referred to as YH pincushion pattern misconvergence. The color CRT disclosed in the related art corrects this type of misconvergence using a four-pole coil. FIG. 5 is a view of such a four-pole coil, seen from the front of the screen. Here, a four-pole coil 4 includes coils 5 and 8, and U-shaped cores 6 and 7. The U-shaped cores 6 and 7 are arranged in opposition on the side of the deflection yoke nearer to the electron gun, so that the electron beams pass between the two cores 6 and 7. When a vertical deflection current is passed through the coils 5 and 8 after being rectified by a diode, force is exerted on the electron beams B and R emitted from the left and right of the electron gun, pushing them away from the vertical axis V, and thereby correcting YH pincushion pattern misconvergence.
In recent years, color CRTs with a virtually flat screen and a wide deflection angle have become increasing commonplace. In such CRTs, the distance the electron beams travel to reach the screen after being emitted from the electron gun varies markedly for each point on the screen surface. This results in increased raster distortion. Of this raster distortion, that which occurs when the top and bottom edges of the raster area scanned by the electron beams bow inward is referred to as top/bottom pincushion distortion, and is conventionally corrected by attaching magnets to the deflection yoke. FIG. 6 is a view of a deflection yoke to which magnets have been attached, seen from in front of the display screen. Magnets 10 and 13 are attached to the front surface of an insulating frame 11 of a deflection yoke 9 at the top and bottom, and a horizontal deflection coil 12 is mounted on the inner surface of the insulating frame 11. When viewed from in front of the display screen, the magnets 10 and 13 are arranged so that the north pole of the magnet 10 is on the right side and the south pole on the left side, while the south pole of the magnet 13 is on the right side and the north pole on the left side. FIG. 7 illustrates magnetic flux generated by the magnets 10 and 13. If the magnets 10 and 13 are arranged in this fashion, forces F are applied to the electron beams according to Fleming""s Law, as shown in FIG. 7, thereby correcting the top/bottom pincushion distortion.
However, a horizontal component Mh of the magnetic fields generated by the magnets 10 and 13 grows weaker at points further away from the magnets. FIG. 8A is a graph plotting Mh against the horizontal axis H. If a point at the center of the horizontal axis H is taken as an origin O, line 14 showing component Mh is symmetrical about the origin O, growing smaller and sloping down more steeply as it moves further away from the origin O. FIG. 8B shows forces Fr, Fg and Fb received by electron beams R, G and B. When the electron beam G coincides with the vertical axis V, in other words when it is positioned so as to correspond to the origin O of the horizontal axis H, vertical deflection forces Fr and Fb are equal, and vertical deflection force Fg is larger than both vertical deflection forces Fr and Fb. When the electron beam R is further away from the origin O than the electron beam B, however, the vertical deflection forces received by the electron beams are such that Fb greater than Fg greater than Fr. Conversely, when the electron beam B is further away from the origin O than the electron beam R, the vertical deflection forces received are such that Fb less than Fg less than Fr. As a result, when a magenta line is displayed horizontally, the misconvergence shown in FIG. 9 is caused. In this type of misconvergence, the red component R (solid line) and the blue component B (broken line) of the magenta line diverge away from each other. This is known as PQV barrel pattern misconvergence.
Although the magnetic field generated by the magnets 10 and 13 relieves barrel distortion of the vertical deflection field, this in turn causes YH pincushion misconvergence to worsen. This misconvergence is so severe that correcting it using a four-pole coil as in the related art increases PQH red right pattern misconvergence. FIG. 10 shows PQH red right pattern misconvergence. In this type of misconvergence, when two magenta lines are displayed vertically on the left and right sides of the display screen, as shown in the drawing, the red component R (solid line) of the magenta line veers to the right and the blue component B (broken line) to the left. Components R and B tend to diverge markedly towards the corners of the display screen. Note that in the drawing, D1 is a distance at which the red component R and the blue component B are furthest apart, and the severity of PQH red right pattern misconvergence can be expressed using this distance D1.
An object of the invention is to provide a color CRT of the type that has become popular in recent years, with a virtually flat screen and a wide deflection angle, and in particular, to provide a color CRT with superior picture quality, that corrects convergence by correcting pincushion distortion at the top and bottom of the raster area using magnets.
The color CRT of the invention has the following structure in order to achieve the above object. A color cathode ray tube (CRT) uses a self-convergence method, has magnets for correcting top/bottom pincushion distortion, and includes the following. A vertical deflection coil generates a first correction field distorted in a barrel shape. A four-pole coil is arranged on a side of a deflection yoke nearer to an electron gun, and generates a second correction field to correct YH barrel pattern misconvergence. Here, the strength of the second correction field varies according to an amount of vertical deflection applied to electron beams emitted by the electron gun.
If the above structure is used, PQV barrel pattern misconvergence generated by magnets can be corrected. YH pincushion pattern misconvergence, which could not be corrected in the related art, is over-corrected to YH barrel pattern misconvergence, and this misconvergence can then be corrected by the four-pole coil. At the same time, PQH red right pattern misconvergence generated when the vertical deflection field is distorted in a barrel shape can also be corrected.
The following structure may be used in order to distort the vertical deflection field in a barrel shape. The vertical deflection coil includes a first coil part and a second coil part connected in series. The first coil part has coil sections with a larger winding angle than a winding angle of coil sections in the second coil part. The first and second coil parts are connected in parallel respectively to first and second impedance elements, and the first correction field may be distorted in the barrel shape by making an impedance of the second impedance element larger than an impedance of the first impedance element. Alternatively, the first correction field may be distorted in the barrel shape by having a greater number of turns in the second coil part than in the first coil part.
Furthermore, the four-pole coil should preferably have the following structure. Three horizontally aligned electron beams are emitted by the electron gun. Here, the second correction field may be generated by the four-pole coil so as to apply an inward horizontal force to each outer electron beam of the three horizontally aligned electron beams. The strength of the second correction field applied to the electron beams is at a maximum when the amount of vertical deflection applied to the electron beams is at a maximum, and at a minimum when the amount of vertical deflection experienced by the electron beams is zero. Furthermore, the four-pole coil may be connected to the vertical deflection coil via a peripheral circuit. The peripheral circuit includes a series circuit in which two resistors are connected in series, two diodes each having a cathode connected respectively to either end of the series circuit, and two variable resistors, each connected respectively to an anode of one of the two diodes at one end, and to one end of the four-pole coil at the other end. Here, the other end of the four-pole coil may be connected to a node at which the two resistors in the series circuit are connected, and the series circuit may be connected in series to the vertical deflection coil. In addition, the four-pole coil may include two coils connected in series. Each of these two coils is wound around one of two U-shaped cores. The U-shaped cores are arranged with corresponding ends in opposition, and the electron beams pass between the opposed U-shaped cores.
Furthermore, VCR misconvergence generated when the vertical deflection field is distorted in a barrel shape can be corrected by using the following structure. The CRT may include a coma correction coil, arranged on the side of the deflection yoke nearer to the electron gun, and used to generate a third correction field to correct vertical coma residual (VCR) misconvergence. Here, a strength of the third correction field may vary according to the amount of vertical deflection applied to the electron beams. Furthermore, the force applied to the electron beams by the third correction field may be applied in a same orientation as the vertical deflection. The forces applied to the outer electron beams may be of equal strength, while a force applied to a central electron beam is greater than the forces applied to the outer electron beams. The strength of the third correction field applied to the electron beams is at a maximum when the amount of vertical deflection applied to the electron beams is at a maximum, and at a minimum when the amount of vertical deflection experienced by the electron beams is zero. The coma correction coil may include two coils that are connected in series, and connected in series to the vertical deflection coil. Each of these two coils is wound around one of two U-shaped cores. The two U-shaped cores are arranged in opposition, and the electron beams pass between the two opposed U-shaped cores.
In addition, a structure such as the following may be used. A color cathode ray tube (CRT) uses a self-convergence method, has magnets for correcting top/bottom pincushion distortion, and includes the following. A magnetic substance, which is either one normally or strongly magnetic, may be arranged on the side of the vertical deflection coil nearer to an outer surface of a glass tube to distort a vertical deflection field in a barrel shape. A four-pole coil may be arranged on a side of a deflection yoke nearer to an electron gun to correct YH barrel pattern misconvergence by generating a second correction field. The strength of the second correction field varies according to an amount of vertical deflection applied to electron beams emitted by the electron gun. Even if such a structure is used, the vertical deflection field can still be distorted in a barrel shape, and so misconvergence can be corrected as above, provided that such a structure includes a four-pole coil and a coma correction coil.